When damage occurs to a joint socket, for example the acetabulum, of a patient, it is common practice to implant an artificial joint socket. Such joint sockets are known in various designs. On the one hand, they can have a multi-part design, for example composed of an outer joint cup (outer cup) and an inner joint cup (inner cup). The outer cup in this case serves for embedding the joint socket in a bone, while the inner cup, which is inserted into the outer cup, forms a bearing surface for a corresponding joint head. On the other hand, one-part artificial joint sockets with only one cup are known. Joint sockets of both types have been used in clinical practice for some time.
If a joint socket is implanted as a pre-assembled unit, for example composed of an outer cup and of an inner cup or with only one cup, it is referred to as a so-called monobloc socket. Joint sockets of this kind reduce the risk of an inner cup coming loose from the outer cup. They are very stable, but their design and the necessary thickness of the respective materials means that they are relatively bulky.
This has the result that, even in the case of quite minor damage to the natural joint socket, for example the acetabulum, of a patient, especially when the cartilage is damaged and the bone intact, quite a lot of material has to be removed when preparing the bone for the implantation. In young patients in particular, this means that, when preparing the bone to receive the artificial joint socket, quite a lot of bone substance is already lost at an early stage, and, consequently, there is no longer so much bone substance available if follow-up surgery proves necessary at a later stage.
A further problem with conventional joint cups is that, on account of the necessary wall thickness of the joint cup, the joint ball is relatively small by comparison with the natural joint. This has the disadvantage of altering the characteristics of the ball joint. Moreover, on account of the small joint ball, very high loads occur at points in the joint. Lastly, the thickness of the wall of artificial joint cups presents a problem, particularly in patients requiring the implantation of a very small joint cup.
EP 1 025 815 A1 discloses an artificial joint cup provided for implantation in a corresponding cavity in the bone of a patient and having a wall thickness of ca. 2.5 mm to 3 mm. Said joint cup is produced from metal, in particular a cobalt-chromium alloy. Moreover, both the inner surface and the outer surface of said joint cup can be coated with different materials.
However, in a joint cup of this kind, the small wall thickness means that the required mechanical stability of the cup structure can be achieved only with difficulty when using metal. Moreover, depending on the choice of the material for the joint head, the sliding behavior of the latter on the inner surface of the metallic cup presents a problem, as a result of which the inner surface of the joint cup in some cases has to be coated with a further non-metallic material. However, applying a particularly smooth coating to the inner surface of the joint cup can be achieved only with considerable effort. A further problem lies in embedding the cup in a stable position in a bone of a patient. A stable hold cannot always be ensured. Moreover, the cup material, in particular on the outer surface, is not always biocompatible.